The present invention relates to a method for preventing polymer scale deposition on the reactor walls in the polymerization of an ethylenically unsaturated monomer such as vinyl chloride and styrene and a coating composition used therefor.
Ethylenically unsaturated monomers are polymerized in several types of polymerization procedures including suspension polymerization, emulsion polymerization, solution polymerization, gas-phase polymerization and bulk polymerization. One of the difficult problems common to all of these polymerization procedures in the prior art is the deposition of polymer scale on the reactor walls and the surfaces of other equipments such as the stirrer and the like coming into contact with the monomer under polymerization.
That is, when an ethylenically unsaturated monomer is polymerized in either one of the above mentioned polymerization procedures, the polymer is deposited more or less on the walls of the reactor and the surfaces of the stirrer and other equipments coming into contact with the monomer to form scale thereon so that various disadvantages are unavoidable such as the decrease in the yield of the desired polymer and lowered cooling capacity of the polymerization reactor as well as degraded quality of the polymer product due to the polymer scale eventually entering the polymer product as coming off the reactor walls. In addition, such polymer scale must be removed taking a great deal of time and labor in order to prepare the polymerization reactor for the next run of the polymerization. The scale removing works by man power present a very serious problem of safety against human body because the polymer scale usually contains a considerably large amount of the unreacted monomer absorbed therein which is sometimes toxic as is a matter of very serious concern in recent years in the case of vinyl chloride in particular.
A number of methods have been proposed hitherto to prevent the polymer scale deposition on the reactor walls. Exemplary of such methods, in particular, for the suspension polymerization of vinyl chloride are a method in which the reactor walls and the surfaces of the stirrer and the like are coated, prior to polymerization, with a polar organic compound such as amine compounds, quinone compounds, aldehyde compounds and the like or a dye or pigment (see, for example, Japanese Patent Publication Nos. 45-30343 and 45-30835), a method in which the walls and surfaces are coated with a polar organic compound or a dye having been treated with a metal salt (see Japanese Patent Publication No. 52-24953), a method in which the coating material is a mixture of an electron donor compound and an electron acceptor compound (see Japanese Patent Publication No. 53-28347) and a method in which the coating material is an inorganic salt or an inorganic complex compound (see Japanese Patent Publication No. 52-24070).
These prior art methods by providing a coating layer on the reactor walls are indeed effective at least to some extent but the effectiveness of a single coating treatment is not so lasting that the scale preventing power of a coating is rapidly lost when polymerization runs are repeated 5 to 6 times or more after a coating treatment. Therefore, it has been eagerly desired to develop an industrially feasible method for the prevention of polymer scale deposition on the reactor walls in which the effect of a single coating treatment is lasting over repeated polymerization runs.
Further, the above mentioned coating methods are effective only in the polymerization runs in which the polymerization initiator is an azo compound or an organic peroxide having a long-chain alkyl group insoluble or hardly soluble in water while the desired effect is not exhibited or greatly reduced when, in the suspension polymerization of vinyl chloride, the polymerization initiator is an organic peroxide which is soluble in the monomer and simultaneously has a relatively high solubility in water to exceed, for example, 0.2% by weight at 20 .degree. C. Such an initiator is exemplified by di-2-ethoxyethylperoxy dicarbonate, bis-3-methoxybutylperoxy dicarbonate, di(butoxyethylperoxy) dicarbonate and the like. This problem virtually prohibits the industrial use of the organic peroxides of such a type as the polymerization initiator notwithstanding the advantages obtained by the use thereof in the quality of the polyvinyl chloride resin products including the absence of the phenomenon of incipient coloring of the resin in the course of molding as well as good thermal stability of the resin. Therefore, it would be greatly advantageous to develop a method for effectively preventing polymer scale deposition on the reactor walls in the suspension polymerization of vinyl chloride even by use of an organic peroxide of the above described type as the polymerization initiator.
Further, in contrast to the suspension polymerization of vinyl chloride which is usually performed in a stainless steel-made polymerization reactor, the emulsion or latex (co)polymerization of styrene or a combination of styrene, butadiene and, in some cases, acrylonitrile is performed usually in a glass-lined polymerization reactor solely due to the problem that no efficient method is known for the polymer scale prevention on the walls of a stainless steel-made polymerization reactor in the polymerization of such a monomer or combination of monomers. Needless to say, glass-lined reactors are less desirable in the industrial polymerization because, even by setting aside the expensiveness in comparison with stainless steel-made ones, the heat transfer through glass-lined reactor walls is poor and the glass-lining layers are susceptible to mechanical damages in addition to the low availability of glass-lined reactors of a large capacity. Therefore, it would be advantageous to develop an efficient method for the polymer scale prevention in the polymerization of styrene and the above mentioned monomers in a stailess steel-made polymerization reactor